A surface plasmon sensor which detects the refractive index of a metal film surface through surface plasmon resonance (resonance of incident light with vibration of an electron in a metal microparticle) is used mainly for research purposes in the field of biotechnology because of high sensitivity and lack of the need for a marker.
A general sensing method using such a sensor condenses light via a prism to cause the light to come incident on a metal film provided on one surface of the prism and detects light reflected from the metal film, thereby analyzing the refractive index of the surface of the metal film from an angle of incidence at which the light is absorbed. Generally, an adsorption layer which adsorbs a particular molecule is provided in advance at the metal film, and the refractive index is converted into the concentration of the molecule.
However, a complicated apparatus composed of a light source, a lens, a prism, and the like is needed to perform the sensing method. The need for accuracy at the time of assembly of the apparatus, strict temperature control that prevents the apparatus from changing over time, correction of a deviation in a sensing result, and the like entails high cost and increased apparatus size. Additionally, high-accuracy detection at the molecular level is difficult for the above-described apparatus.
Aside from this, a method using a resonator is proposed for small-sized and high-sensitivity detection.
For example, Japanese Unexamined Patent Application Publication No. 2007-537439 discloses a sensor which incorporates a micro-resonator in a part of a planar waveguide and detects a change in spectral response due to a change in the refractive index of a surface of the micro-resonator. The micro-resonator is a resonator for surface plasmon waves. The micro-resonator includes a thin metal layer, and a reflecting section uses a distributed Bragg reflector (DBR) with a periodic structure.
Japanese Patent No. 04224641 discloses a localized surface plasmon sensor in which a metal microparticle layer sized to excite localized surface plasmon resonance is formed on an end face of an optical fiber, and a molecular layer of a molecule complementary to a detection target molecule is formed on a surface of the metal microparticle layer. The localized surface plasmon sensor detects the detection target molecule adsorbed or bonded to the complementary molecule using a change in light reflected or scattered from the end face of the optical fiber.
However, the sensor disclosed in Japanese Unexamined Patent Application Publication No. 2007-537439 detects a detection target by converting light from a light source into surface plasmon, causing the surface plasmon to react with the detection target with the micro-resonator resonating, and reconverting the surface plasmon into light, and loss of light intensity occurs in the conversion from light into surface plasmon. Additionally, since a resonator with a length of 2 to 10 microns is utilized, a surface plasmon wave as a damped wave is lost inside the resonator, and improvement in sensitivity cannot be expected. Moreover, there is a limit to a size reduction due to the need for a separate light source.
The sensor disclosed in Japanese Patent No. 04224641 does not use a resonator and include low sensitivity. When light from a light source is coupled to the optical fiber in the sensor, loss of light intensity occurs.